As an improvement in luminous efficiency becomes more important in recent years, LEDs (Light Emitting Diode) are being widely used in backlights provided in a display device and lighting apparatuses as a light source that saves more energy than electric bulbs or fluorescent lamps. For these purposes, energy efficiency is very important.
However, LEDs, especially, gallium nitride LEDs are easily broken due to electrostatic discharges. That is, the LEDs withstand only a small reverse voltage. In order to prevent such a problem, there has been disclosed a technique in which Zener diodes are connected to LEDs in an inverse-parallel connection (see Patent Literature 1, for example).
In such a configuration that employs the Zener diodes, when an excessive voltage is applied in a forward direction, an excessive current is bypassed due to Zener breakdown. In the meantime, when an excessive voltage is applied in a reverse direction, the Zener diodes serve as normal forward diodes to bypass an excessive current. In this way, the LEDs are protected from the excessive voltage in either direction. Further, a forward voltage to be applied to the LEDs is smaller than a Zener breakdown voltage of the Zener diodes. Even if the forward voltage is applied to the LEDs, no current flows into the Zener diodes, thereby resulting in that no energy loss occurs.
As another technique to prevent the above problem, there has been disclosed a technique in which resistors are connected to respective LEDs in parallel (see Patent Literatures 2 and 3, for example).
FIG. 13 illustrates a circuit configuration of an LED combination lamp 1000 described in Patent Literature 2. The LED combination lamp 1000 is configured such that a plurality of LEDs 1100 connected in series are connected to respective resistors (Rb) 1200 in parallel. With the configuration, even in a case where a given LED 1100 is disconnected, a corresponding resistor 1200 works as a bypass resistor, thereby making it possible to prevent that the other LEDs 1100 are turned off. Further, the configuration makes it possible to prevent deterioration of the LEDs 1100.
However, in order that the bypass resistor carries out its purpose, it is necessary to supply, to the bypass resistor, a current sufficient to turn on the other LEDs 1100 that are not disconnected. In view of this, it is necessary to use, as the resistors 1200, resistors having a low resistance. This arises such a problem that the current flowing through the bypass resistor causes large energy loss.
Further, Patent Literature 3 discloses a semiconductor light emitting device in which a plurality of LEDs are connected to respective variable resistors in parallel or in series so that respective currents flowing through the plurality of LEDs are adjustable. The semiconductor light emitting device also has such a problem that large energy loss occurs because an electric resistance of the variable resistors should be low.
Patent Literature 4 discloses, as an exemplary format ion of resistors to be connected to LEDs, an LED array in which a plurality of LEDs are connected in series to respective thick-film resistive elements.